I. Field of the Invention
This invention relates to flow sensing devices. Such devices may be used in a variety of applications; one particular field in which they find application is process control. A flow sensing device may be required to provide a digital (two-state) signal, indicating whether the flow is above or below a threshold value; alternatively, it may be required to provide an analogue signal indicative of the actual value of the flow. The present invention can be used in either of these situations.
II. Description of the Prior Art
Flow sensing devices have already been proposed in which some kind of vane is movably supported in the flow to be sensed; the force exerted by the flow impinging on the vane gives a measure of the flow rate. In these prior proposals, the vane has usually been mounted on an arm extending radially from a rotatable shaft at or near the side of the conduit carrying the flow, so that the vane can be moved in an arcuate path by the action of the flow. The shaft transmits the movement of the vane to some kind of measuring device outside the conduit, which device provides the output signal, and so a seal is required around the shaft where it leaves the conduit, to prevent the leakage of process fluid, but without impeding the rotation of the shaft.